title: Cyanamide route to calcium-manganese oxide foams for water oxidation.
abstract: In nature, photosynthetic water oxidation is efficiently catalysed at a protein-bound μ-oxido Mn4Ca cluster. This cluster consists of earth abundant, non-toxic elements and serves as a paragon for development of synthetic catalysts. In this study we developed porous calcium-manganese oxides with a unique foam-like nanostructure prepared via a facile and robust synthetic route using cyanamide as a porogen. A series of such oxide foams annealed at different temperatures was characterized by TEM, SEM, XRD, N2 physisorption, and X-ray absorption spectroscopy (XAS) in order to correlate crystallinity, atomic structure, surface area and oxidation state of the materials with catalytic activity. Some of the resulting Ca-Mn oxides show high activity as catalysts for water oxidation in the presence of cerium(iv) ammonium nitrate as a non-oxo transfer oxidant. An amorphous calcium-manganese-oxide foam with 130 m(2) g(-1) surface area and Mn oxidation state of +3.6 was identified to be most active; its activity is superior to previously reported Ca-Mn oxides. At the atomic level, this material shares structural motifs with the biological paragon as revealed by dual-edge XAS at the Mn and Ca K-edge. Rather than nanostructure and surface area, the atomic structure of the Ca-Mn oxide and the extent of structural order appear to be crucial determinants of catalytic activity. Fully disordered low-valent Mn materials as well as high-valent but crystalline Mn-Ca oxides are unreactive. Highly disordered variants of layered manganese oxide with Ca and water molecules interfacing layer fragments are most reactive.
authors.affiliation: Technische Universität Berlin, Department of Chemistry, Hardenbergstr. 40, 10623 Berlin, Germany. arne.thomas@tu-berlin.de.
authors.forename: Elham
authors.initials: E
authors.lastname: Baktash
authors.forename: Ivelina
authors.initials: I
authors.lastname: Zaharieva
authors.forename: Marc
authors.initials: M
authors.lastname: Schröder
authors.forename: Caren
authors.initials: C
authors.lastname: Goebel
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.forename: Arne
authors.initials: A
authors.lastname: Thomas